Electronically controlled ingestible capsules can be used to provide therapeutic or diagnostic treatment during traversal of the gastrointestinal alimentary tract. For example, an ingestible capsule can acquire diagnostic data as it traverses the gastrointestinal tract. If used for therapeutic treatment the capsule can be provided with an electronically controlled medicament delivery system. To that end it typically contains a battery, a medicine reservoir, a medicine dosing and delivering device, such as a pump, and electronic circuitry for controlling medicine delivery and optionally for two-way communication. The capsule moves through the intestines by the peristaltic movement of the muscles along the gastrointestinal tract.
The capsule comprises a sensor, e.g. a pH sensor for determining the location within the gastrointestinal tract. When the pill passes the pylorus, a pH step of about 3-4 is made. If the pill is used for controlled medication release this moment can be used as a trigger for controlled release in the small intestines. The drift velocity in the small intestines is dependent on the time of day. During or after a meal the intestines are more active than in between meals. Releasing medication directly after passage through the pylorus delivers the medication at the beginning of the jejunum releasing 2-3 hours later somewhere at the end of the jejunum and the beginning of the ileum releasing after 4-5 hours later it delivers the medication at the end of the ileum. As the value of the pH slowly increases from the beginning of the jejunum to halfway the small intestines the pH can also be used for triggering controlled release of medication. When passing through the ileocaecal valve (valvula Bauhini), the pH drops by 1-1.5 pH. This step can be used for triggering controlled release of medication at the beginning of the ascending colon. The motility of the large intestines is less dependent on mealtime and follows its own rhythm.
The pH value can for example be measured by using an electrochemical cell comprising an indicating electrode whose potential is directly proportional to the pH, a reference electrode whose potential is independent of the pH, and the liquid to be measured. The measured voltage is indicative for the pH of the liquid. The system is calibrated by placing the electrodes in solutions of known pH and measuring the corresponding voltages of the cell.
The electrodes are placed in a sample of the liquid to be measured. The voltage is then measured and the pH is determined using the calibration data.
The reference electrode used in pH measurements usually is a silver wire coated with silver chloride embedded by an electrolyte solution, e.g. of potassium chloride (KCl). The reference electrode must be in electrical contact with the pH electrode through the sample. Therefore the container with reference electrode and the electrolyte solution must communicate electrically with the sample, e.g., via a porous liquid junction, e.g., of ceramic, wood or plastic which allows ions to pass between the fill solution and the sample.
US 2004/0106849 discloses an electronic pill for diagnostic purposes comprising an ISFET type pH sensor.
Since the capsule must be swallowable, the capsule must be as small as possible. Therefore, there is only very limited space for the sensor and its components. However, the smaller the volume of electrolyte, the more it is susceptible of drying which limits the shelf life of the capsule.